Pairing of lacO tandem repeats in Arabidopsis thaliana nuclei requires the presence of hypermethylated, large arrays at two chromosomal positions, but does not depend on H3-lysine-9-dimethylation.

Fluorescent chromatin tagging by the lacO operator/lac repressor system in Arabidopsis thaliana is useful to trace distinct chromatin domains in living cells. Nevertheless, the tandem repeats of the tagging system may alter the spatial organisation of chromatin within nuclei by increasing homologous pairing as well as association with heterochromatin. Efficient homologous pairing occurs if lacO repeat arrays of ∼10 kb are present at two loci, either on the same chromosome or on different chromosomes. DNA hypomethylation of lacO repeats results in reduced homologous pairing. Because, in plants, DNA methylation can serve as a signal for H3-lysine9-dimethylation (H3K9me2), and subsequently for non-CG-context DNA methylation, SET-domain histone methyltransferase and chromodomain dna methyltransferase 3 (cmt3) mutations were introgressed. In suvh4 suvh5 suvh6 and cmt3 mutants, H3K9me2 associated with lacO repeats is diminished, but homologous pairing persists. Thus, neither H3K9me2 nor CMT3-mediated non-CG methylation are required at wild-type level for homologous pairing of lacO repeat loci.